It is common knowledge that the performance of a shaft seal of any type is greatly affected by the physical properties of the fluids which it seals and that fluid properties, such as density, viscosity, specific heat, etc. can affect the performance of the seal. For example, rubbing seals of the segmented circumferential type for sealing between a housing and a rotating member have been in use for many years as main bearing shaft seals on aircraft jet engines and on gas compressors (U.S. Pat. No. 2,908,516). The segments of such seals, when sealing liquids, tend to "surf-board" on the fluid film between the segments and the rotating member, causing the segments to be lifted out of contact with the surface on which they are intended to rub resulting in excessive leakage. This has limited such seals to use largely in dry gas applications. In such applications it is known to reduce rubbing loads by giving the rubbing seal a configuration producing a decrease in fluid film thickness in the direction of the rotation of the rotary member to produce a positive hydrodynamic lift to space the seal away from the rotating member a small distance, for example 0.005 mm. It is also known from U.S. Pat. No. 3,516,678 to maintain a limited leakage shaft seal in a balanced position by providing a film leakage path from the high pressure side to the low pressure side of the seal which has a step down in thickness providing a pressure gradient along the path which varies with the film thickness. This is a hydrostatic seal.
Heretofore, rubbing shaft seals which were required to seal both gas and liquid, alternatively, posed very difficult problems, especially when rubbing speeds were considerable. The difficulties resulted from the differences in the nature of the fluids. Gases, in general, have lower viscosity, lower density and lower heat capacity per unit volume than liquids. Limits of rubbing load to prevent overheating are much lower with gas seals than with liquid seals. The face drag with liquid seals for a given rubbing velocity is much higher than the face drag with gas seals. A consequence of the face drag is that the loadings on rotational locking devices and other restraints are very much higher in liquid seals than in gas seals for the same rubbing velocity. Heretofore, to overcome the friction resulting from these higher loadings, the axial springing had to be higher with liquid seals than with gas seals. When the seal ring is sealing against a ring secured to a shaft even a small out-of-flatness of about 30 millionths of an inch can cause high lift-off of the seal face when liquids are being sealed due to the hydrodynamic effect in the convergent fluid film regions causing excessive leakage unless the face load is increased sufficiently to prevent such lift-off. Nutational oscillation of the seal face is a common problem. These factors have made the use of rubbing seals to seal both gases and liquids, alternatively, unsatisfactory.
The above discussed problems are solved by this invention.